1. Field of the Invention
The present invention relates to a charging apparatus that is charged using an electric generator driven by an internal combustion engine, and more particularly, to a vehicle charging apparatus suitable for a hybrid automobile.
2. Description of the Related Art
In an automobile in the related art using the internal combustion engine alone as the power source, an electric generator is mounted to charge a battery, and the battery is charged by converting part of a torque generated by the engine to electric power. The electric generator normally outputs an AC (alternate current), which is converted to a DC (direct current) using a rectifier and the DC is supplied to the battery. While there are cases where a charging voltage of the battery, that is, a DC voltage outputted from the rectifier is automatically adjusted to a constant predetermined voltage by a circuit in the electric generator or the rectifier, a technique of making an output current of the rectifier or an output voltage of the electric generator variable by a control device has been proposed.
As a technique of making an output current of a rectifier variable by a control device, there is a power generating unit disclosed (for example, see Japanese Published Patent Application No. H11-46456 (Patent Document 1)). According to the technique disclosed herein, the control device is configured to enable a supply of electric power from a battery to a coil in an electric generator by switching an output current of a rectifier, so that an output current of the rectifier is made variable by controlling a magnetic flux interlinking with the coil by controlling the switching timing of the rectifier.
Also, as a technique of making an output voltage of an electric generator variable, there is an output control device for an AC electric generator disclosed (for example, see Japanese Published Patent Application No. H06-197470 (Patent Document 2)). According to the technique disclosed herein, the control device is configured to be capable of controlling a field current of an electric generator, so that an output voltage of a rectifier is made variable by controlling the field current. In this case, the control device measures a current output voltage of the rectifier and performs feedback control for the field current to coincide with a given target output voltage.
Either technique makes an output voltage of the rectifier variable with the aim of making a torque that the electric generator extracts from the engine variable in achieving better acceleration performance and preventing rotation fluctuation of the engine.
Meanwhile, a hybrid automobile generates electric power from kinetic energy of the engine or the vehicle and performs regeneration to accumulate electric power and power running to add a torque to the drive shaft by driving a motor using the accumulated electric power. In short, in the hybrid automobile, an electric storage device to drive the motor is mounted. As the electric storage device, a secondary battery, such as nickel hydride and lithium-ion batteries, and a capacitor are used chiefly. The capacitor has lower internal resistance than the secondary battery and therefore has an advantage that large electric power can be supplied to the motor in a short time. However, a voltage of the capacitor drops as a charging amount of the capacitor decreases. Also, in a case where output electric power of the electric generator is large and the capacitor is charged with the electric power outputted directly from the rectifier, a large current flows into the capacitor. This may possibly cause deterioration or damage of the capacitor.
As a countermeasure for such an inconvenience, Japanese Published Patent Application No. 2000-278807 (Patent Document 3) discloses a power supply system of a hybrid electric automobile. According to a technique disclosed herein, a charging current to a capacitor is limited by interposing a resistor between an inverter as a rectifier (hereinafter, referred to simply as the inverter) and the capacitor in series or by interposing a resistor or an inductor between an electric generator and the inverter.
Also, Japanese Published Patent Application No. 2005-269825 (Patent Document 4) discloses a hybrid system. According to a technique disclosed herein, a large current is prevented from flowing to a capacitor by connecting a step-up and -down chopper in series between an inverter and the capacitor or by connecting a constant current circuit in parallel. The step-up and -down chopper is a device that generates a command value of a charging current for a voltage of the capacitor when the capacitor is charged and steps down a charging voltage in response to the generated command value of the charging current. The constant current circuit is a circuit that adjusts a charging current to the capacitor to a constant value.
However, when the techniques disclosed in Patent Document 1 and Patent Document 2 to be applied to the battery are applied to a hybrid automobile using the capacitor as the electric storage device, there is a possibility of deteriorating or damaging the capacitor. Because the hybrid automobile is configured to drive the motor using electric power accumulated in the capacitor during the power running, the voltage of the capacitor may possibly drop to the vicinity of 0 V. Accordingly, when the hybrid automobile shifts later from the power running to the regeneration, charging is started while the voltage of the capacitor has dropped to the vicinity of 0 V.
According to the power generating unit disclosed in Patent Document 1, however, when the voltage of the capacitor has dropped to the vicinity of 0 V, electric power used to adjust the magnetic flux interlinking with the coil in the electric generator cannot be supplied from the capacitor to the electric generator. Hence, because the charging current cannot be lowered, the capacitor is charged with the maximum output voltage of the electric generator determined by the rotation number of the electric generator. Accordingly, in a case where charging is started at a specific rotation number or higher, there is a problem that a large current flows into the capacitor and may possibly deteriorate or damage the capacitor.
Also, in the output control device of the AC electric generator disclosed in Patent Document 2, the target charging voltage is set with the aim of preventing rotation fluctuation caused by an abrupt change in a load torque of the engine during electric power generation. However, no upper limit is set to a voltage difference between the target charging voltage and the current voltage of the capacitor. Hence, there is a possibility that the voltage difference is such that generates a current that deteriorates or damages the capacitor. In particular, when the engine is running at a high rotation speed and low load, an output torque of the engine is sufficiently large in comparison with a torque consumed by the electric generator, which raises a problem that there is a possibility of increasing a voltage difference.
Further, Patent Document 3 and Patent Document 4 have a problem that efficiency is lowered by a loss generated at the resistor, the inductor, the step-up and -down chopper, or the constant current circuit interposed to prevent a large current from flowing into the capacitor.